1. Effect of varied lengths of darkness given before continuous illumination, and that of dark-break of continuous light as a function of the time of its application, on the flower formation in a long-day duckweed,Lemna gibbaG3, were studied.The results obtained suggested a rhythmic change in sensitivity to darkness, i.e., a cycle of 36 hr-period consisting of 12 hr of sensitivity and the following 24 hr of insensitivity. The inhibition by darkness (12–36 hr) given before the start of, or by dark-break (12, 24 hr) inserted in, the induction period involved an extension of the induction period, but not a slow-down of the rate of flower formation. The dark-break given after the induction period, however, suppressed the rate of flower production in proportion to the length of the darkness.2. The inhibition of flowering by darkness given in the darksensitive phase was cancelled by a relatively brief light period inserted in the darkness.3. Relation between the rhythm and the length of induction period was discusse
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